Thin-film deposition apparatus

ABSTRACT

A thin-film deposition apparatus having a holder and a plurality of racks. Each of the racks has a surface which has the conformation of a portion of the surface of a sphere. Means is provided for rotatably mounting the racks on the holder in such a manner that said surfaces of the racks lie generally on the surface of a common sphere. Means is provided for rotating the holder upon an axis centrally disposed with respect to the racks and for rotating the racks about their own axes of rotation with respect to the holder.

United States Patent Mahl [54] THIN-FILM DEPOSITION APPARATUS [72] Inventor: Gunard 0. B. Mahl, San Francisco, Calif.

[73] Assignee: Carl Hermann Associates, Inc., Menlo Park, Calif.

[22] Filed: Oct. 7, 1969 [21] Appl. No.: 864,377

[52] US. Cl. ..118/48, 118/500 [51] Int. Cl ..C23c 11/02 [58] Field of Search ..118/47-49.5, 500, 118/503; 204/298; 117/106-1072, 93.2, 93.1 DH; 219/278; 269/57 [56] References Cited UNITED STATES PATENTS 3,128,205 4/ 1964 lllsley ..l18/49 2,997,979 8/1961 Tassara 18/49 1 Feb. 22, 1972 3,523,517 8/1970 Corbani ..l18/500 3,314,395 4/1967 Hemmer ..1 18/47 2,532,971 12/1950 Van Leer et al.... ...1 l8/49.l X 3,271,561 9/1966 Fiedler et al.... ..2l9/27l 3,437,734 4/1969 Roman et a1 ..l l8/49.5 X

Primary Examiner-Morris Kaplan Attorney-Flehr, Hohbach, Test, Albritton & Herbert ABSTRACT A thin-film deposition apparatus having a holder and a plurality of racks. Each of the racks has a surface which has the conformation of a portion of the surface of a sphere. Means is provided for rotatably mounting the racks on the holder in such a manner that said surfaces of the racks lie generally on the surface of a common sphere. Means is provided for rotating the holder upon an axis centrally disposed with respect to the racks and for rotating the racks about their own axes of rotation with respect to the holder.

. 13 Claims, 9 Drawing Figures PAIENTEDFEBZZ I972 3, 643.625

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SHEET '4 BF 4 INVENTOR. Gunard 0., B. Mahl Attorneys THIN-FILM DEPOSITION APPARATUS BACKGROUND OF THE INVENTION In depositing thin films there is a need for obtaining a very uniform deposition of coating materials on surfaces. This has been particularly true with respect to parts which have irregular surfaces such as parts with grooves, slots, steps and/or windows therein and in which it is desirable to obtain a uniform deposition of material in the slots. There is, therefore, need for a new and improved thin-film deposition film apparatus.

SUMMARY OF THE INVENTION AND OBJECTS The thin-film deposition apparatus consists of a holder and a plurality of racks. Each rack has a surface which has a conformation generally in the form of a portion of a surface of a sphere. Means is also provided for rotatably mounting each of the racks on the holder in such a manner that the surfaces of the racks are disposed generally on the surface of a common sphere. Means is provided for rotating the holder upon an axis centrally disposed with respect to the racks so that the racks rotate about said central axis. Means is also provided for rotating the racks about their axes of rotation with respect to the holder during the time the holder is being rotated.

In general, it is an object of the present invention to provide a thin-film deposition apparatus in which it is possible to obtain a very uniform deposition of the coating materials by utilization of the principle of the cosine law distribution.

Another object of the invention is to provide apparatus of the above character in which a relatively simple and unique double rotation system is utilized.

Another object of the invention is to provide apparatus of the above character in which it is possible to achieve a relatively uniform coating in any grooves, slots or other depressions in the substrates.

Another object of the invention is to provide apparatus of the above character in which the racks can be readily removed for loading and unloading.

Another object of the invention is to provide apparatus of the above character in which very simple means is provided for rotating the racks.

Another object of the invention is to provide apparatus of the above character in which the racks are mounted on a spider assembly which can be removed as a unitary assembly.

Additional objects and features of the invention will appear from the following description in which the preferred embodimentis set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevational view with certain portions 'broken away of a thin-film apparatus incorporating the present invention.

FIG. 2 is an enlarged side elevational view of the deposition apparatus shown in FIG. 1 with the cage and bell jar removed.

FIG. 3 is a still further enlarged side elevational view of a portion of the deposition apparatus shown in FIG. 2.

FIG. 4 is a top plan view looking along the line 3-3 of FIG. 2.

FIG. 5 is a cross-sectional view looking along the line 5-5 of FIG. 2.

FIG. 6 is a cross-sectional view taken along the line 6-6 of FIG. 5.

FIG. 7 is an enlarged detail view of a portion of the apparatus shown in FIG. 5.

FIG. 8 is an enlarged detail view of the clips utilized in conjunction with the racks in the thin-film deposition apparatus.

FIG. 9 is a cross-sectional view taken along the line 9-9 of FIG. 8.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT The thin film deposition apparatus consists of a base plate 11 upon which are mounted a plurality of vertical upstanding support rods 12. The source 13 is of a conventional type and is mounted upon the base plate 11 so that it is centrally disposed on the base plate 11. The source 13 is provided with means for evaporating material placed in the source 13 for deposition on substrates as hereinafter described. A shutter 14 is carried by a rod 16 which is rotatably mounted in a support member 17 carried by one of the support rods 12. A knob 18 is mounted on the rod and is adapted to move the shutter 14 into position in which it overlies the source 13 or into a position in which it is away from the source 13.

A cooling coil assembly 21 of a conventional type is mounted upon the base plate 11 and encircles the source 13 as shown particularly in FIG. 2. A heater assembly 22 is mounted adjacent to the upper extremity of the cooling assembly 21 and is carried by the support rods 12 in a manner hereinafter described.

Means is provided for carrying the substrates which are to be coated in theapparatus and includes a stationary support ring 24 which is mounted upon the upper ends of the support rods or posts 12. The support ring 24 is provided with four pins 26 which are secured to the bottom of the support ring 24 by suitable means such as welding. The pins 26 extend into holes 27 provided in upper ends of the support rods 12 and are retained therein by set screws 28. The support ring 24 is provided with a large opening 31 which has inwardly and downwardly inclined annular wall 32. In addition, the ring 24 is provided with annular recesses 33 and 34 of differing diameters as shown particularly in FIG. 3. The recesses 33 and 34 are generally in the form of right angles in cross section as shown particularly in FIG. 3. The inner recess or groove 33 serves as a track as hereinafter described, whereas the second larger recess 34 serves as a catch or retainer. The support ring 24 is provided with an outwardly facing annular slot 36 which is covered by a strip 37 which is welded thereto so that a fluid passage is formed for cooling water.

The means for carrying the substrates in the apparatus also includes a holder or spider 38 which is provided with a central hub 39 and three downwardly and outwardly curved arms 41 spaced apart which are secured to the central hub and extend radially therefrom. A plurality of racks 43, namely three, are carried by the holder. As can be seen from the drawings, the racks 43 are circular and have at least one surface which has a conformation that corresponds to a portion of the surface of a sphere. In the arrangement shown in the drawings, the three racks 43 are in fact in such a form that they can be considered to be three spherical sections of a large common sphere.

Means is provided for rotating and mounting the racks 43 on the ends of the arms 41 in such a manner that when the outer peripheries of the racks 43 are within the track formed by the recess 33, they will travel in the surface of a sphere as they are rotated or orbited as hereinafter described. Each of the arms 41 carries a split bearing retaining collar 46 which has slidably mounted therein ball bearing assemblies 47. A post 48 is mounted on each of the racks 43 and is mounted in the roller bearing assemblies 47 and is retained therein by a retaining clip 49. Each rack 43 with its bearing assembly 47 can be readily slipped downwardly and outwardly out of the collar 46 to facilitate loading of the rack. The collar 46 is provided with an inwardly extending lip 46a to retain the bearing assembly 47 so that the bearing assembly can only be removed in one direction. A small annular shield 50 is provided on each of the posts 48 to protect the bearing assemblies 47 from the vapor stream.

The bearing support collars 46 are stabilized by means of a stabilizing ring 51 which extends between the same and is secured thereto by suitable means such as welding.

Each of the racks 43 is provided with a plurality of circular openings 53 which are spaced relatively uniformly over the surface of the rack. The openings 53 are generally of the size of the substrates which are to be coated to facilitate loading of the substrates in the rack. A spring clip 54 is provided for holding the substrate in the desired position on the rack. The

nun-u spring clips 54 are formed of a suitable material such as Inconel which has been heat treated so it can withstand the hightemperature conditions encountered during deposition. The spring clip 54, generally U-shaped as shown in FIG. 8, is provided with a pair of spaced end portions 54a which extend through and are secured to the rack adjacent to one of the openings 53. The spring clip is provided with a downwardly depending portion 54b which is yieldably urged into engagement with a clamping member 56 which is secured to the rack 43 at a point adjacent to the other side of the opening 53. The clamping member 56 is provided with a support surface 57 which extends away from the inner curved surface of the rack 43. Thus, there is shown a substrate 58 of a suitable type such as silicon wafer which has one edge of the same clamped between the support surface 57 and the spring clip 54. The clip 54 can hold wafers of various sizes and can even hold pieces of wafers. By bending the clamping member 56, it is possible to hold the wafer or substrate at difierent angles to thereby change the angle of incidence of the vapor stream to the wafer or substrate.

Means is provided for causing rotation of the holder or spider 38 so that it will be rotated about an axis which is centrally disposed with respect to the racks 43. Such means consists of a Z-shaped drive pin 61 which has its lower end free but which is adapted to engage one of the arms 41 as shown particularly in FIG. 3. The other end of the drive pin 61 is mounted in a drive member 62. The drive member 62 is rotatably mounted in a pair of ball bearing assemblies 63 and is provided with a central opening 65 to permit direct source monitoring with suitable monitoring equipment. The ball bearing assemblies 63 are carried by a support collar 64 mounted upon the end of a gusset 66. The gusset 66 is mounted upon a drive support member 67 which is secured to one side of the support ring 24 by suitable means such as welding. The ball bearing assemblies 63 are retained within the collar 64 by a retaining clip 68. A sprocket 69 is formed integral with the upper end of the drive member 62. Thesprocket 69 is driven by a chain 72 and the chain 72 is driven by a sprocket 73. An eccentrically mounted idler sprocket 74 is rotatably mounted upon the pin 76 and mounted in a boss 77 carried by cross member 78 mounted upon the gusset 66.

The sprocket 73 is secured to a drive shaft 81 by a set screw 82. The drive shaft 81 is rotatably mounted in ball bearing assemblies 83 mounted in the upper and lower ends of the drive support member 67 and retained therein by retaining rings 84.

A sprocket 86 is mounted on the lower end of the drive shaft 81 and is secured thereto by a set screw 87. The sprocket 86 is driven by a chain 88 and the chain 88 is driven by a sprocket 89. The sprocket 89 is secured by a set screw 91 to another drive shaft 92. The upper end of the drive shaft 92 is mounted in a ball bearing assembly 93 mounted within a collar 94 secured to the support ring 24. The lower end ofthe shaft 92 is connected by a coupling 96 to another drive shaft 97. The shaft 97 extends through a sealed bearing assembly 98 of a conventional type which is mounted in the base plate 11. A rough vacuum can be applied to the sealed bearing assembly 98 through a connector 99. Another coupling 101 (see FIG. 2) is adapted to be connected to the output shaft 102 ofa gear motor (not shown) so that the shaft 97 is driven at a speed ranging from 0 to 20 rpm.

The heater assembly 22 which is provided consists of a circular plate 106 which is carried by the support rods 12 at a point which is below the support ring 24. Four quartz heaters 107 are mounted upon the upper surface of the plate 106' so that the plate 106 protects the heaters from the evaporation source. The heaters 107 are mounted in brackets 108 carried by the plate 106. Reflectors 109 are mounted on the plate 106 the collar in a predetermined vertical position on the support rod. it has been found that these screws'112 are far'superior to set screws because they do not mar the support rods 12 which permits the collars 111 to be readily adjusted vertically of the support rods.

The thin-film deposition apparatus also includes additional conventional parts as, for example, a glass bell jar 116 and a protective cage 117 (see FIG. 1). Such apparatus also includes means for supplying a vacuum to the bell jar. However, since such means is conventional, they will not be disclosed in detail.

Operation of the thin-film deposition apparatus may now be briefly described as follows. Let it be assumed that the cage 117 and the bell jar 116 have been removed. When this has been accomplished, the holder or spider 38 with the racks 43 mounted thereon can be lifted as a subassembly from the support ring 24. This is possible because there is no direct solid connection between the spider and the drive pin 61. Since the racks and holder can be readily removed, it is very easy to load the substrates into theracks. Let it be assumed that substrates such as silicon wafers are mounted on the racks by use of the spring clips to assume positions such as that shown in FIG. 9.

After the three racks have been loaded, the entire subassembly can then be positioned upon the support ring 24 with the outer lower extremities of the racks engaging the grooved recess 33 and with the weight of the holder 38 upon the racks 43. The bell jar 116 and the cage 117 can be replaced and the apparatus can be pumped down. Thereafter, the heater assembly can be operated if desired to bake out the silicon wafers. During this time the substrates which are to be coated are rotated. This is accomplished by energizing the drive motor which is connected to the coupling 101 which causes rotation of the shaft 81 and the drive member 62. Rotation of the drive member 62 causes rotation of the drive pin 61. The drive pin 61 engages the spider or holder 38 and causes it to rotate on the axis of rotation for the holder or spider which is centrally disposed between the racks 43. At the same time that this rotation is occurring, the racks are rotated about their own axes of rotation because their lower extremities frictionally engage the support ring 24 which causes the same to be rotated. Thus, it can be seen that the drive system is such that a double rotation is simultaneously provided for the substrates. The first rotation or orbital movement is rotation about the central axis for the spider or holder 38 and the second or other rotation is about the axis of rotation which is coincident with the axis of rotation for the rack. The spider assembly rotation and the rack rotation are stabilized by the use of the double bearings.

After thebake-out has been completed, the double rotation of the racks with the substrates carried thereby can be continued. Thereafter, the source can be energized to evaporate material to cause coatings to be deposited upon the substrates carried by the racks. The double rotation which is provided for the substrates in the manner hereinbefore described facilitates obtaining a much more uniform distribution of the coating material on the exposed surfaces of all of the substrates. In addition, it will be noted that since the substrates are placed at an angle with respect to the spherical surface of the racks, the substrates will be presented at different angles to the evaporation source. This change of angle of the substrates is particularly desirable when it is desired to deposit the evaporated material into grooves or slots carried by the substrates. in other words, this apparatus makes it possible to obtain a more uniform distribution of the coating over the entire exposed surface of the substrate and, in addition, makes it possible to obtain a more uniform distribution of the coating material in the windows or slots in the substrate. As pointed out previously, the angle of the chips relative to the rack can be adjusted by bending the retaining members 56 to different angles.

it is apparent from the foregoing that there has been provided a thin-film deposition apparatus which has many advantages. Because of the construction of the holder and the rack mounting upon the holder, the entire subassembly can be removed to facilitate ready loading and unloading of subtnlInA "run strates. In addition, the substrates are rotated about two axes of rotation to obtain much greater uniformity in the coating of the material on the substrates. Also, the substrates are carried at an angle with respect to the surface of the common sphere in which the racks rotate so that the substrates may be presented at continuously varying angles to the source to facilitate a more uniform distribution of the coating in slots or openings provided on the substrates.

Iclaim:

1. In a thin-film deposition apparatus, a vacuum chamber, a stationary support ring in said chamber, a holder in said chamber, a plurality of circular racks in said chamber, each of said racks having at least one surface having the conformation of a portion of the outer surface of a sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere, means in said chamber for rotatably mounting said racks on said holder so that the outer peripheral surfaces of said racks rest upon said support ring and are adapted to travel on said support ring and so that said one surface of each of said racks lies substantially on the outer surface of said common sphere, and means for rotating said holder about an axis centrally disposed with respect to said racks so that said racks travel on said support ring whereby said racks are orbited about an axis which is generally centrally disposed with respect to said racks and are simultaneously rotated about axes which are coincident with their respective axes of rotation.

2. Apparatus as in claim 1 wherein said means for rotating said holder includes a drive pin, said drive pin having a portion thereof extending downwardly so that it is adapted to engage the holder, and means for rotating said drive pin.

3. Apparatus as in claim 2 wherein said holder is provided with three outwardly extending arms and wherein three of said racks are provided and are mounted on the outer extremities of said arms, and wherein said drive pin is adapted to engage one of said arms.

4. Apparatus as in claim 3 wherein a bearing retaining collar is mounted on the outer ends of each of said arms and wherein said means for rotatably mounting each of said racks includes a bearing assembly mounted in said bearing retaining collar and means for securing said racks to said bearings, said racks being removable from said collars by lifting said bearing assemblies out ofsaid bearing collars.

5. Apparatus as in claim 4 wherein said holder and the racks are constructed so that all of the weight of said holder rests upon said racks.

6. Apparatus as in claim 1 wherein each of said racks is provided with a plurality of openings, a spring clip mounted in each of said racks adjacent to each of said openings and extending into the opening, support members mounted on the racks adjacent to said openings in the racks and positioned so that said spring clips are adapted to yieldably engage said support members whereby substrates may be placed between the same and held at angles with respect to the surface of the racks.

7. Apparatus as in claim 1 together with the heater assembly disposed below said support ring, said heater assembly including a plurality of heaters and means for protecting the heaters from the evaporation source.

8. Apparatus as in claim 7 together with an evaporation source disposed so that it is generally in alignment with the axis of rotation for the holder.

9. Apparatus as in claim 7 together with vertical support rods, means for mounting said support ring upon said rods and means for mounting said heater assembly on said support rods, said means for protecting the heaters including a plate and wherein said means mounting said heater assembly on said support rods includes a plurality of collars slidably mounted on said support rods and screws having eccentric heads mounted in said collars and adapted to be rotated so that their heads frictionally engage the support rods to hold the collars in redeterm ined positions on the support rods.

0, In a thin-film deposition apparatus, a vacuum chamber,

a holder mounted in said chamber, a plurality of racks disposed in said chamber, each of said racks having at least one surface having the conformation of a portion of the outer surface ofa sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere, means in said chamber rotatably mounting said racks on said holder to permit rotation of each of said racks about its own axis and for positioning said racks so that said racks are mounted in such a manner that said one surface of each of said racks lies substantially in the outer surface of said common sphere, and means for causing rotation of said holder about an axis centrally disposed with respect to said racks and to cause orbital movement of said racks about said centrally disposed axis and also for causing rotational movement of said racks about their own individual axes of rotation while at all times maintaining said one surface of each of said racks substantially in the outer surface of said common sphere.

11. Apparatus as in claim 10 wherein said holder is provided with three outwardly extending arms and wherein three of said racks are provided with one of the racks mounted on each of said arms.

12. In a thin-film deposition apparatus, a vacuum chamber; a holder mounted in said chamber; a plurality of racks disposed in said chamber, each of said racks having at least one surface having the conformation ofa portion of the outer surface ofa sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere; means in said chamber rotatably mounting said racks on said holder to permit rotation of each of said racks about its own axis and for positioning said racks so that said racks are mounted in such a manner that said one surface of each of said racks lies substantially in the outer surface of said common sphere; means for causing rotation of said holder about an axis centrally disposed with respect to said racks and to cause orbital movement of said racks about said centrally disposed axis and also for causing rotational movement of said racks about their own individual axes of rotation while at all times maintaining said one surface of each of said racks substantially in the outer surface of said common sphere; and means for supporting a series of substrates to be coated on each of said racks comprising a series of angularly adjustable clamping members mounted on each of said racks at predetermined locations, each of said clamping members projecting from said one surface thereof, and a series of spring members mounted on each of said racks in association with said clamping members for holding substrates to be coated on said racks at predetermined angles relative to the source of deposition material so that uniform coating of said substrates may be insured.

13. The apparatus of claim 12 in which said racks are three in number and are carried by said holder at equally spaced locations relative to said centrally disposed axis. 

1. In a thin-film deposition apparatus, a vacuum chamber, a stationary support ring in said chamber, a holder in said chamber, a plurality of circular racks in said chamber, each of said racks having at least one surface having the conformation of a portion of the outer surface of a sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere, means in said chamber for rotatably mounting said racks on said holder so that the outer peripheral surfaces of said racks rest upon said support ring and are adapted to travel on said support ring and so that said one surface of each of said racks lies substantially on the outer surface of said common sphere, and means for rotating said holder about an axis centrally disposed with respect to said racks so that said racks travel on said support ring whereby said racks are orbited about an axis which is generally centrally disposed with respect to said racks and are simultaneously rotated about axes which are coincident with their respective axes of rotation.
 2. Apparatus as in claim 1 wherein said means for rotating said holder includes a drive pin, said drive pin having a portion thereof extending downwardly so that it is adapted to engage the holder, and means for rotating said drive pin.
 3. Apparatus as in claim 2 wherein said holder is provided with three outwardly extending arms and wherein three of said racks are provided and are mounted on the outer extremities of said arms, and wherein said drive pin is adapted to engage one of said arms.
 4. Apparatus as in claim 3 wherein a bearing retaining collar is mounted on the outer ends of each of said arms and wherein said means for rotatabLy mounting each of said racks includes a bearing assembly mounted in said bearing retaining collar and means for securing said racks to said bearings, said racks being removable from said collars by lifting said bearing assemblies out of said bearing collars.
 5. Apparatus as in claim 4 wherein said holder and the racks are constructed so that all of the weight of said holder rests upon said racks.
 6. Apparatus as in claim 1 wherein each of said racks is provided with a plurality of openings, a spring clip mounted in each of said racks adjacent to each of said openings and extending into the opening, support members mounted on the racks adjacent to said openings in the racks and positioned so that said spring clips are adapted to yieldably engage said support members whereby substrates may be placed between the same and held at angles with respect to the surface of the racks.
 7. Apparatus as in claim 1 together with the heater assembly disposed below said support ring, said heater assembly including a plurality of heaters and means for protecting the heaters from the evaporation source.
 8. Apparatus as in claim 7 together with an evaporation source disposed so that it is generally in alignment with the axis of rotation for the holder.
 9. Apparatus as in claim 7 together with vertical support rods, means for mounting said support ring upon said rods and means for mounting said heater assembly on said support rods, said means for protecting the heaters including a plate and wherein said means mounting said heater assembly on said support rods includes a plurality of collars slidably mounted on said support rods and screws having eccentric heads mounted in said collars and adapted to be rotated so that their heads frictionally engage the support rods to hold the collars in predetermined positions on the support rods.
 10. In a thin-film deposition apparatus, a vacuum chamber, a holder mounted in said chamber, a plurality of racks disposed in said chamber, each of said racks having at least one surface having the conformation of a portion of the outer surface of a sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere, means in said chamber rotatably mounting said racks on said holder to permit rotation of each of said racks about its own axis and for positioning said racks so that said racks are mounted in such a manner that said one surface of each of said racks lies substantially in the outer surface of said common sphere, and means for causing rotation of said holder about an axis centrally disposed with respect to said racks and to cause orbital movement of said racks about said centrally disposed axis and also for causing rotational movement of said racks about their own individual axes of rotation while at all times maintaining said one surface of each of said racks substantially in the outer surface of said common sphere.
 11. Apparatus as in claim 10 wherein said holder is provided with three outwardly extending arms and wherein three of said racks are provided with one of the racks mounted on each of said arms.
 12. In a thin-film deposition apparatus, a vacuum chamber; a holder mounted in said chamber; a plurality of racks disposed in said chamber, each of said racks having at least one surface having the conformation of a portion of the outer surface of a sphere which can be generated within the confines of said chamber, said surfaces of said racks lying substantially on the outer surface of a common sphere; means in said chamber rotatably mounting said racks on said holder to permit rotation of each of said racks about its own axis and for positioning said racks so that said racks are mounted in such a manner that said one surface of each of said racks lies substantially in the outer surface of said common sphere; means for causing rotation of said holder about an axis centrally disposed with respect to said racks and to cause orbital movement of Said racks about said centrally disposed axis and also for causing rotational movement of said racks about their own individual axes of rotation while at all times maintaining said one surface of each of said racks substantially in the outer surface of said common sphere; and means for supporting a series of substrates to be coated on each of said racks comprising a series of angularly adjustable clamping members mounted on each of said racks at predetermined locations, each of said clamping members projecting from said one surface thereof, and a series of spring members mounted on each of said racks in association with said clamping members for holding substrates to be coated on said racks at predetermined angles relative to the source of deposition material so that uniform coating of said substrates may be insured.
 13. The apparatus of claim 12 in which said racks are three in number and are carried by said holder at equally spaced locations relative to said centrally disposed axis. 